An example of a prior art multiplex communication device by a spread spectrum communication system effecting high speed data communication is indicated in FIGS. 9 and 10.
FIG. 9 represents a transmitter, in which reference numeral 1 is a serial-parallel converter; 2-1.about.2-n are multipliers; 3-1.about.3-n are PN code generators; 4-1.about.4-n are BPSK modulators; and 5 is an adder.
In the transmitter described above, inputted high speed data A re converted into parallel data sets (B1), (B2), . . . , (Bn) by the serial-parallel converter 1. Each of the parallel data sets (B1), (B2), . . . , (Bn) is inputted to one of the inputs of each of the multipliers 2-1, 2-2, ..., 2-n, respectively. On the other hand, each of different PN codes (C1), (C2), . . . , (Cn) outputted by the PN code generators 3-1, 3-2, . . . , 3-n is inputted to the other input of each of the multipliers 2-1, 2-2, . . . , 2-n, respectively. Outputs (D1), (D2), . . . , (Dn) of the multipliers 2-1, 2-2, . . . , 2-n are inputted to the BPSK modulators 4-1, 4-2, . . . , 4-n, respectively, to modulate a high frequency carrier signal (E). High frequency signals (F1), (F2), . . . , (Fn) are outputted by the BPSK modulators 4-1, 4-2, . . . , 4-n, respectively, to be inputted to the adder 5. A spread spectrum signal (G) n-fold multiplexed is outputted by the adder 5 to be transmitted.
FIG. 10 represents a receiver, in which 7-1.about.7-n are convolvers; 8-1.about.8-n are multipliers; 9-1.about.9-n are PN code generators; 10-1-10-n are detectors; and 12 is a data demodulator.
In the receiver described above, received signal (H) is distributed to the convolvers 7-1, 7-2, . . . , 7-n to be inputted to one of the inputs of each of them.
On the other hand, each of PN codes (K1), (K2), . . . , (Kn) outputted by the PN code generators 9-1, 9-2, . . . , 9-n is applied to one of the inputs of each of the multipliers 8-1, 8-2, . . . , 8-n, respectively. A high frequency carrier signal (L) is inputted to the other input of each of the multipliers 8-1, 8-2, . . . , 8-n. Each of outputs (I1), (I2), . . . , (In) of the multipliers 8-1, 8-2, . . . , 8-n is inputted to the other input of each of the convolvers 7-1, 7-2, . . . , 7-n, respectively.
Outputs (J1), (J2), . . . , (Jn) of the convolvers are inputted to the detectors 10-1, 10-2, . . . , 10-n. At this time, a correlation spike is produced at the output of each of the convolvers from each of data channels with a same timing. Outputs (M1), (M2), . . . , (Mn) of the detectors 10-1, 10-2, . . . , 10-n are inputted to the data demodulator 12. Reproduced data (N) are outputted by the data demodulator 12.
In the prior art multiplex communication device described above, synchronization of the carrier is required and in addition a plurality of convolvers (or matched filters) serving as correlators are necessary.
For this reason a construction described below has been proposed as a spread spectrum multiplex communication device capable of demodulating data by means of a single correlator.
FIGS. 4 and 5 represent a transmitter and a receiver, respectively, constituting an example of the spread spectrum (SS) communication device described above.
As indicated in FIG. 4, the transmitter is composed of a serial-parallel converting circuit 101, a group of selectors 102, a group of delay devices 103, an adder 104, a PN code generator 105, a high frequency carrier generator 106 and a multiplier 107.
As indicated in FIG. 5, the receiver is composed of a convolver 201 serving as a correlator, a multiplier 202, a high frequency carrier generator 203, a PN code generator 204, a high pass filter (HPF) 205, an amplifier 206, a detector 207, a binary pulse generating circuit 208, a sounder pulse detecting circuit 209, a sampling pulse generating circuit 210, an information detecting circuit 211 and a parallel-serial converting circuit 212.
Now the operation of the device described above will be explained. At first, in the transmitter, transmission data a are converted into signals of a plurality of channels by the serial-parallel converting circuit 101. Here, for the sake of simplifying the explanation, it is supposed that the number of the channels is N. Further the transmission data a are converted into signals having a lower transmission speed by the serial-parallel converting circuit 101 at its outputs. The transmission data a are converted into parallel data having a transmission speed equal to 1/N of the transmission data a or a transmission speed arbitrarily lower than it. The spread spectrum modulation (SS modulation) is effected, depending on the polarity of each of the channels from the serial-parallel converting circuit 101.
The SS modulation described above is carried out e.g. according to either one of following two systems.
1. CSK (Code Shift Keying) system: system, by which two kinds of PN codes (PN1 and PN2) are outputted selectively, depending on the polarity of the data (signal),
2. OOK (On Off Keying) system: system, by which it is selected, depending on the polarity of the data (signal), whether a PN code should be outputted or not.
In order to realize the SS modulation operation by the two systems described above, the spread spectrum modulator is constructed by the PN code generator 105 for generating the PN codes (PN1 and PN2) and the group of selectors 102 for effecting the selection described above, depending on each of the outputs of the serial-parallel converting circuit 101. The output of each of the selectors in the spread spectrum modulator is inputted to each delay device of the group of delay devices 103. SS modulated signals (information channels), for which arbitrary amounts of delay different from each other are set, are obtained from the outputs of the delay devices by using the phase of the PN code (here it is supposed to be PN1) of the sounder channel serving as a data demodulating synchronization signal as the reference. This aspect is indicated in FIG. 6. FIG. 6 represents differences between the CSK system and the OOK system for different delay amounts (.tau..sub.1 .about..tau..sub.4), in the case where it is supposed that there are four information channels 11.about.14. S represents the sounder channel. It represents further that the transmission speed of the transmission data is transformed into a transmission speed lower than it of the different information channels. Here it is transformed into 1/4 of the original transmission speed. N SS modulated signals of the information channels obtained by the different delay devices and the signal of the sounder channel are added by the adder 104 in an analogue manner (multiplexing) and the output of the adder 104 is multiplied by the output of the high frequency carrier generator 106 by means of the multiplier 107 to obtain a multiplexed SS signal.
Then, in the receiver, the multiplexed SS signal is inputted to one of the inputs of the convolver 201 as a received signal.
A PN code high-frequency-modulated by multiplying a PN code (here the PN code (PN1), which is in a relation inverted in time with respect to the PN code (PN1) used in the transmitter, is used) by the output of the high frequency carrier generator 203 by means of the multiplier 202 is inputted to the other input terminal of the convolver as a reference signal.
The convolver carries out a correlation operation between the received signal and the reference signal to obtain a high frequency correlation output.
In FIG. 7, correlation peaks separated in time, corresponding to the different PN codes of the different information channels, which are in different phase relations with respect to the phase of the PN code of the sounder channel explained, referring to FIG. 6, are obtained.
Here a state is indicated, where correlation peaks, which are self correlations for the sounder channel and all the information channels, are obtained.
Consequently, in the case where no correlation is obtained for either one of the CSK system and the OOK system (CSK system . . . mutual correlation, OOK system . . . no correlation), no correlation peaks are produced.
Although the device is described for the case where convolvers are used for the correlators, no problem is produced also when matched filters are used therefor.
However the place where the reference signal is generated is replaced by a pattern on the matched filters and therefore it is unnecessary.
Then the output of the convolver is detected by the detector 207 through the high pass filter 205 and the amplifier 206 and converted into a signal in the base band information band to obtain a pulse train of logic level in the binary pulse generating circuit 208.
Further, in the binary pulse generating circuit 208, a threshold value is set so as to be able to separate the correlation peaks from the spurious level in the optimum manner.
Since the correlation output corresponding to the sounder channel produces always periodical correlation peaks, the reference time signal is obtained by detecting the correlation peaks by the sounder pulse detecting circuit 209.
The reason why such a time signal serving as a reference is necessary for making the spread spectrum code synchronization in the usual DS-SS system unnecessary.
That is, the device described above realizes not a system, by which data are reproduced by effecting phase synchronization between the PN code of the received signal and the reference signal on the convolver, but an asynchronous system, in which a mere code synchronizing process is omitted.
Sampling pulses for sampling the correlation pulses corresponding to the different information channels are produced in the sampling pulse generating circuit 210 on the basis of the reference time signal, which is the output of this sounder pulse detecting circuit 209.
In the case where convolvers are used for the correlators, since the received signal and the reference signal correspond to each other, the correlation peaks are produced at the gate delay time/2. That is, in this way, the correlation outputs corresponding to the delay amounts (.tau..sub.1 .about..tau..sub.4) of the different information channels with respect to the phase of the PN code of the sounder channel on the transmitter side indicated in FIG. 3 are produced, separated in time also by about (.tau..sub.1 /2.about..tau..sub.4 /2).
Consequently the sampling pulses are produced, taking this property described above into account. In this way the data train of each of the information channels is reproduced by sampling the correlation outputs corresponding to the different information channels in the information detecting circuit 211 on the basis of the sampling pulses.
The data obtained here are those having a transmission speed identical to the lower transmission speed after the serial-parallel conversion on the transmitter side.
Then the transmission data are reproduced by converting these N parallel data trains into serial data in the parallel-serial converting circuit 212.
The outline of this series of operations is indicated in FIG. 8.
Since the device described above, in particular the transmitter, uses a group of delay devices, a group of selecting circuits, an adder, etc., it has a problem that the circuit construction is complicated and has a large scale.